Band Gap Engineering toward Semimetallic Character of Quinone-Rich Polydopamine

Abstract

Semiconductor|melanin interfaces have received increasingly more attention in the fields of photocatalysis and applied electrochemistry because of their facile synthesis, unique electrical properties, and strong capability toward photosensitization. In this work, we describe the electropolymerization of quinone-rich polydopamine (PDA) on the surface of hydrogenated TiO 2 nanotubes with enhanced photoactivity in the visible spectrum. PDA is deposited uniformly on the nanotube walls, and the chemical structures of the resulting PDA layer strongly depend on the pH of the supporting electrolyte. The film thickness varies in the range of 2−8 nm depending on the number of electropolymerization cycles. Optical and electrochemical experiments coupled with density functional theory simulations revealed strong evidence of a semimetallic character of the junction with a broad distribution of midgap surface states induced by PDA. As a result of the nanotube modification, a 20-fold increase in the photocurrent response is observed. Quantum efficiency measurements show that the enhancement occurs mainly at wavelengths between 500 and 550 nm. Additionally, nonlinear electrochemical impedance spectroscopy experiments strongly suggest that, in the TiO 2 |PDA junction, PDA behaves as a set of redox mediators distributed at the surface rather than as a semiconducting polymer. This concept might be crucial for understanding the electronic properties of semiconductor|melanin junctions.

Laser-Induced Graphitization of Polydopamine on Titania Nanotubes

“…The former can be interpreted as a noncovalent interaction of the PDA structural units with the surface layer of the TiO 2 rather than the formation of titanium carbides. This statement has been elucidated in our previous work and supported by DFT calculations . The latter is a direct consequence of introducing PDA, which contains C–N bonds of different order, depending on the oxidation state of each unit.…”

“…The latter is a direct consequence of introducing PDA, which contains C–N bonds of different order, depending on the oxidation state of each unit. Such bonds are also reported on the nitrogen spectrum at 397.8, 399.9, and 401.8 eV associated with aromatic (imine) R-N-R groups, aliphatic R-N-R groups, and tetraalkylammonium (NR 4 + ) groups, respectively. ,− However, one should consider that noisy N 1s spectra of TNT IgPDA 532, originating from the low N concentration, affect deconvolution certainty. Additionally, another doublet appeared on the Ti spectrum corresponding to the TiO-C interaction parallel to the data on the C spectrum.…”

“…This statement has been elucidated in our previous work and supported by DFT calculations. 56 The latter is a direct consequence of introducing PDA, which contains C−N bonds of different order, depending on the oxidation state of each unit. Such bonds are also reported on the nitrogen spectrum at 397.8, 399.9, and 401.8 eV associated with aromatic (imine) R-N-R groups, aliphatic R-N-R groups, and tetraalkylammonium (NR 4 + ) groups, respectively.…”

“…Such PDA electropolymerized on the surface of titania nanotubes leads to 20-fold photocurrent enhancement through photosensitization as it behaves as a net of spatially distributed redox mediators in electrochemical experiments with redox mediators. 56 In this work, electropolymerized PDA is modified using a pulsed Nd:YAG laser with 365 and 532 nm wavelengths and varying pulse parameters. Titania nanotubes are chosen as a substrate for PDA deposition (prelaser graphitization) for several reasons.…”

“…In our previous work, we have shown the alternative picture to think of quinone-rich PDA as a semimetallic entity. Such PDA electropolymerized on the surface of titania nanotubes leads to 20-fold photocurrent enhancement through photosensitization as it behaves as a net of spatially distributed redox mediators in electrochemical experiments with redox mediators …”